1. Field of the Invention
This invention relates to a non-invasive method of oximetry in which light contacts an area of skin surface of a subject and is then detected to determine oxygen saturation of the patient's blood, and to an apparatus for carrying out the method. The light which contacts the area can either then pass through the skin of the subject before reaching the detector, or can be reflected from the area to be directed at the detector. More specifically, this invention relates to such a method wherein the rate of change of light intensity is determined to thereby determine oxygen saturation, and to an apparatus for carrying out the method.
2. Statement of the Prior Art
Oximetry methods are used to determine oxygen saturation of a subject's blood, i.e., the percentage of oxygenated hemoglobin in the blood. Such methods may be of the invasive or the non-invasive types. The non-invasive type can be further subdivided into a transmittance method, and a reflectance method. With both the transmittance and reflectance method, a source of light is directed at an area of skin surface of the subject. In the transmittance method, the light passes through the skin of the subject and is then detected by the detector. In the reflectance method, the light is reflected by the area and is then directed at the detector.
In presently known methods of ear oximetry, a light source is directed at one side of the ear lobe or pinna (hereinafter referred to as the ear lobe) and a light detector on the opposite side of the ear detects the intensity of light transmitted through the ear lobe or pinna. Oximetry methods are classified a either relative or absolute.
In the relative methods, a reference is necessary, and saturation is determined relative to the reference. As is well known, the amount of light absorbed by the ear as light is transmitted through it is a function of the attenuation due to skin, muscle, fat, cartilage, etc. of the ear as well as the attenuation due to blood in the ear. The attenuation due to blood is itself dependent on the amount of oxygenated hemoglobin in the blood.
In the absolute method, light at two different frequencies is used, and, advantage is taken of the knowledge that the degree of absorption of red light at a certain frequency is different for oxygenated vs. deoxygenated blood. However, as regards infra-red light at a certain frequency, the degree of absorption is the same for both oxygenated and deoxygenated blood. By measuring absorption at red and infra-red light, oxygen saturation can be determined.
One approach of the absolute method is to provide a transducer which can squeeze the ear tightly to provide a "bloodless ear." The amount of light absorbed by the bloodless ear is measured, and the transducer is then adjusted so that the ear is no longer squeezed and blood can once again flow in the ear. Light is again transmitted through the ear lobe under the second condition, and the difference in the amount of light absorbed under the two conditions is used as an indication of the amount of oxygenated hemoglobin in the blood.
This approach has the disadvantages that, no matter how tight the ear lobe is squeezed, there is still some blood left, so that oxygen saturation determined in this fashion may be inaccurate. Further, the approach is clumsy, and therefore not often used, and, in addition, this approach does not take into account the differences of absorption due to differences in the non-blood tissue in the light path.
Other disadvantages of this method are that results may be affected by such variables as the depth of blood in the ear lobe, and differences in total hemoglobin concentration in the blood.